In order to deflect an electron beam--or other charged particle beam (e.g., ion beam)--in a raster scanning manner, ordinarily at least four plates are required. Charged-particle-beam control circuitry applies a controllable voltage separately to each of the plates. However, in many practical applications, advantageously eight (or more) plates in an "octapole arrangement" are used, because the octapole (or higher) arrangement provides more uniform deflection fields (lower fringing fields). In this way, the deflections of the charged particle beam are more accurately controlled--i.e., do not suffer from the distortions otherwise caused by the relatively large fringing fields produced by smaller numbers of plates. If desired, more than eight plates can be used, such as sixteen or as many thirty-two plates in a multiple-pole arrangement, in order to reduce the fringing fields still further.
When using an octapole (or higher pole) arrangement, in prior art the voltage applied to each of the plates is supplied by a general purpose digital computer acting through several separate multiplying digital-to-analog converters ("DACs") and operational amplifiers ("Op-Amps") for each plate. The circuitry of each DAC and Op-Amp is analog, and therefore the circuitry suffers from ordinary noise and instability problems. As the number of plates increase, the noise generated in the circuitry becomes a critical problem because of the increased complexity of the required analog circuitry, and hence the noise increases with the number of plates. Moreover, as the number of plates increase, stability of voltages delivered to the plates also becomes a critical problem, because the stability deteriorates as the complexity of the analog circuitry increases.
Relevant to this subject is U.S. Pat. No. 4,053,740, entitled "Video Game System," issued to Lawrence David Rosenthal on Oct. 11, 1977. That patent, however deals only with deflection of the charged particle beam by means in the x direction controlled by an RC voltage decay applied to a first pair of plates and in the y direction by a second pair of plates. The voltage applied to the second pair of plates is generated, among other circuitry, by means of an accumulating Arithmetic Logic Unit ("ALU"). However, the circuitry taught by this patent is quite limited insofar as the kinds of deflections that it produces.
It would be desirable to have multiple-pole charged particle beam deflection control circuitry that alleviates one or more problems of prior art.